Without a doubt! I totally agree with the comments above, season 1 had the absolutely best cast and all the characters were so interesting - I mean, how can you replace Charlie with Daniel?! xD Plus it wasn't that science fictionesque.
Season 5 is only fourth in my list.

Season five is pretty much head and shoulders above season 1 for me. Were just eyeballs deep into mythology, the storytelling is faster paced with more exciting turns all the time. Plus season five was just pretty ballsy. The most bold of all the seasons and about as bold as any season of any network show have ever been.

Season 1 was a very strong 'base' to the series, but I'm afraid it is just that; a base.
So many new characters have come long after along with some stunning revelations set way back in season 1 (ala Locke in the nightmare with black eyes) only now to make sense in season 5. Along with so many other character developments that have made are Losties grow and evolve into more than just caricatures but real solid people we care for. Just look at season 5 Sawyer in contrast to angry season 1 Sawyer!